1. Field of the Invention (Technical Field),
The invention relates specifically to the discovery that certain low cost thermoplastics such as Polyvinylchloride (PVC) and chlorinated polyvinylchloride (CPVC) can be excellent materials for the construction of high and ultra high vacuum components. Prior art vacuum use of these materials has been restricted to vacuum pump exhaust lines and rough vacuum. It has been conclusively determined through the use of residual gas analysis that the prior bias against the use of these materials in this service relates to surface impurities that are either a residue of manufacture, a non vacuum compatible lubricant or a joining adhesive, not continuous outgas or permeation of the PVC or CPVC plastic material itself. Subsequently, when these materials are exposed to high vacuum pressures, the surface impurities are gradually eliminated. This type of surface contamination would, by its presence, render any material unsuitable for high and ultra high vacuum service. The invention uses an outgas reduction process to demonstrate the capability of PVC and CPVC plastic compounds to be suitable for use in the construction of high and ultra high vacuum systems, where vacuum pressures from 9.times.10.sup.-4 torr to 1.times.10.sup.-11 torr are required. The present invention has also determined the relative purity level of the vacuum processed plastic components through the use of residual gas analysis. It should be noted that not all thermoplastic materials exhibit adequate high and ultra high vacuum performance. It is expected that through the inventions evaluation of thermoplastics that other cost effective materials may be found in addition to PVC and CPVC.
2. Background Art
The vacuum industry has considered low cost thermoplastics such as PVC, or CPVC to be unsuitable as a material of construction for high and ultra high vacuum components. This relates primarily to the perceived outgassing rate of such plastic materials and, secondly, to the perceived rate at which gases are able to permeate through the plastic material into the contained vacuum pressure space. "Outgassing" is the tendency of materials to release gases over time. The rate at which materials give off gas, or outgas, is particularly a problem in high and ultra high vacuum applications where systems are almost exclusively fabricated from expensive polished stainless steel components which have low outgassing characteristics. Metal components for high and ultra high vacuum piping are often supplied with a helium leak rate certificate but little or no information is currently supplied with vacuum piping components which specifies the outgas/permeation rate that these components produce. Low cost PVC and CPVC components are typically used in the construction of liquid transfer piping systems. Because these components are manufactured in very high quantity they are low in cost to end users.